1. Field of the Invention
The present invention relates to a method for forming a gate insulating layer and nitrogen density measuring method thereof, in which a thin layer is not grown by a conventional method using NO or NO2 and by which a transistor having enhanced electrical characteristics can be fabricated without employing a separate ion implantation process in a manner of providing parameters for enhancing perfection of the transistor via nitridation measurement.
2. Discussion of the Related Art
Generally, an insulating layer of a transistor has been reduced in thickness as increasingly higher degrees of device integration are achieved. A silicon oxide layer, which becomes thinner, is vulnerable to initial insulation failure or degradation of electrical characteristics such as time-delay insulation breakdown, transistor characteristics, etc. Moreover, Boron (B) as a dopant of a P+ polysilicon electrode penetrates the silicon oxide layer into a substrate during annealing. Thus, the electrical characteristics are degraded because dangling bonds exist at an interface between the oxide layer and a basic silicon layer considerably affect the electrical property of the oxide layer.
To overcome such a problem, methods of improving the electrical characteristics in a manner of oxidizing a surface of a silicon layer and forming a nitrogen-based diffusion barrier within the silicon oxide layer have been proposed. For instance, there is an annealing method using NO or NO2. The annealing method needs a process temperature of at least 1,000° C. to improve the electrical characteristics, thereby causing a problem of thermal budget to a layer. The thermal budget refers to a lowering of threshold voltage (or other device characteristic) that is adversely influenced by exposing one or more layers of a device to a high temperature. Besides, there are various methods of injecting nitrogen into an insulating layer such as a method of growing an insulating layer using NO, a method using plasma, etc.
In a lamp heating type sheet handler disclosed in Japanese Patent Laid-Open No. 93-251428, dry oxidization is carried out on a silicon substrate to form a silicon oxide layer. The silicon oxide layer is then processed with an oxidative gas containing nitrogen to form a silicon oxide-nitride layer.
FIG. 1 is a graph of temperature vs. time for forming an oxide-nitride layer according to a related art.
Referring to FIG. 1, a silicon substrate is loaded in a process chamber. A silicon oxide layer is then formed by performing oxidization on the silicon substrate in a manner of heating the substrate at 1,000° C. during a section-L in FIG. 1 by inputting an oxidative gas excluding nitrogen, e.g., dry O2, to the process chamber. And, a section-K is a temperature rise section. In doing so, the substrate is heated by an infrared lamp and rapid heating is used at 50˜200° C./sec. A temperature sustain time for oxidization at 1,000° C. is set to about 10 sec. And, a process pressure for oxidization is set to about 760 Torr. Once a vacuum state is provided to the process chamber while the silicon substrate remains at about 1,000° C., an oxidative gas containing nitrogen is inputted to the process chamber during a section-M so that the silicon oxide layer can turn into a silicon oxide-nitride layer. In doing so, the oxidative gas containing nitrogen is at least one selected from the group consisting of NO, NO2, and N2O. And, a temperature sustain time for oxidation-nitridation is set to about 30 seconds. Moreover, a process pressure for oxidation-nitridation is set to about 760 Torr. Sections J, N, O and P relate to other processing steps not further described herein.
Korean Patent Application Laid-Open No. 2003-0042873 discloses a quantification method of nitrogen density in an interface between a silicon substrate and a silicon oxide layer, in which density of nitrogen contained in the interface between the silicon substrate and the silicon oxide layer is quantified. In an analysis sample in which a silicon oxide layer is stacked on a silicon substrate, a strength of SiN ion and strength of SiON are measured per vertical depth toward a bottom of the silicon substrate from a surface of the silicon oxide layer. From the strengths of the ions, half maximum widths of SiN and SiON are calculated, respectively. By squaring to add the half maximum width of SiN and the half maximum width of SiON, the density of nitrogen existing in the interface between the silicon substrate and the silicon oxide layer is quantified. In doing so, the ion strength is measured by a secondary ion mass spectrometer. Since the matrix effect can be corrected by the above method, the nitrogen density of the interface between the silicon substrate and the silicon oxide layer can be precisely measured.
However, as recognized by the present inventor, in the related art method of forming the thin gate insulating layer, a short channel effect occurs and boron (B) penetrates the silicon oxide layer to the substrate.